1. Field of the Invention
This invention relates to a printed tone control method and a representative point determining apparatus for use in controlling ink feeding rates of a printing machine.
2. Description of the Related Art
In order to perform proper printing with a printing machine, it is necessary to control ink feeding rates properly. For controlling the ink feeding rates, it has been conventional practice to measure densities of control strips with a densitometer and determine from density data whether the ink feeding rates are proper or not. However, the density data from the control strips alone is not necessarily sufficient for attaining a proper color tone and the like for a picture area.
For this reason, a print quality measuring apparatus is used which provides control data for controlling the ink feeding rates of a printing machine. The control data is produced by comparing an image on reference paper and an image on an actual print.
The reference paper is also called proof paper, and serves as a reference indicating a color tone of finished prints to obtain proper prints. Printing paper actually printed is also called sampling paper which is extracted by the operator from a discharge station of a printing machine at certain intervals during a printing operation. The printing is considered proper when the color tone on the sampling paper substantially coincides with the color tone on the reference paper.
It has become conventional in recent years to include a print quality measuring apparatus in a printing machine. This construction enables tone control to be carried out in parallel with a printing operation without requiring the operator to extract sample prints.
Where such a print quality measuring apparatus is used, it is difficult to attain an agreement in color tone and the like in all areas between the image on the reference paper and the image on the actual print. For this reason, use is made of a representative point having a color that characterizes a picture on the print, as disclosed in Japanese Unexamined Patent Publications Nos. 2004-50609 and 2004-351814. The representative point is used in order to perform color control of the image, and is set to the image automatically by the apparatus or by the operator.
Although the representative point set in this way has a color that characterizes the picture on the print, this point is not necessarily suitable for color measurement. In time of color measurement, the inks of Y (yellow), M (magenta), C (cyan) and K (black) are measured by R (red), G (green) and B (blue). It is therefore difficult to separate accurately the colors of a point including a large quantity of black ink. It is also difficult to effect accurate color tone control, depending on an overlap of a plurality of colors and a dot percentage.